New biodegradable elastomers for drug and therapeutic protein delivery

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Abstract

Over the past few years, and with the tremendous advances in recombinant protein technology, cytokines and other therapeutic proteins have emerged as a promising and effective strategy in immunotherapy of cancer. As such, much of the current research has focused on many aspects of developing new drug delivery systems which are capable of targeting and controlling the release of cytokines for the treatment of cancer. Most of those efforts however were directed towards the systemic administration of cytokines which is often limited by side effects and the necessity to administer these therapeutic proteins in a clinical setting. There are several reasons for the attractiveness of the regional delivery approach of cytokines in cancer therapy. It is generally recognized that loco-regional delivery localizes the cytokines actions and activities into the vicinity of tumors, reducing the dose required, providing uniform delivery and can result in an improved therapeutic outcome with much less side effects or toxicity. Different strategies have been utilized to deliver interleukin 2 (IL-2) to the tumor site including gene therapy and polymeric drug delivery systems. Some of the polymeric delivery systems investigated to date include liposomes, hydrogels and biodegradable microspheres made from polylacticacid, polyglycolicacid and their copolymers. An extensive evaluation of the different delivery approaches utilized for IL-2 has been reviewed in chapter 1. These delivery strategies have inherent problem in maintaining the required therapeutic activity over time and/or in preserving their content of the loaded IL-2 and /or maintaining its stability during their formulation techniques. -- The main objective of our studies was to formulate a new biodegradable polymeric drug delivery system capable of providing a constant and sustained IL-2 delivery rate. The new polymeric system will be designed in a manner in which the IL-2 protein particles could be easily dispersed, into this synthesized polymeric system, in a protein friendly formulation condition. This shall provide an environment to maintain IL-2 stability and activity while within the delivery device and prior to it being released. On the other hand the release, which is undergone by utilizing the osmotic driven release mechanism, shall facilitate complete release of the protein before the device eventually degrades and becomes bioabsorbable. In addition, the use of a slowly hydrolysable copolymer would reduce or eliminate the production of an acidic environment within the device until the vast majority of the loaded protein is released, before any significant reduction in the mechanical properties of the biomaterial occurs due to its hydrolysis. -- These new polymeric biomaterials are based on the poly(diol-tricarballylate) (PDT) photocrosslinked elastomers. The synthesis was carried out by the polycondensation reaction of tricarballylic acid and alkylene diols, followed by acrylation and visible light photo-curing under ambient temperature and solventless conditions, which is described in chapter 3. Chapter 4 describes the in vitro evaluation of the osmotic-driven release mechanism by a release study using the papaverine hydrochloride, as a model for a water soluble drug. The examinations of in vitro cytotoxicity, in vivo biocompatibility and biodegradability were also conducted to assess the elastomers' biological performance after long term contact with lung epithelial cell line and soft subcutaneous tissue of Sprague-Dawley rats, the details of which is described in chapter 5. Eventually, different IL-2 loaded devices have been formed by homogenous distribution of the co-lyophilized IL-2 and stabilizing/osmotic excipient particles throughout the acrylated prepolymer which is then photocrosslinked. These experiments are described in chapter 6. The in vitro IL-2 release studies in phosphate buffer saline of pH 7.4 were undertaken to demonstrate the proposed osmotic mechanism and to determine the time frame for the release. The activity of the released IL-2 was also examined in vitro using C57BL/6 mouse cytotoxic T lymphocyte. Chapter 7 is devoted to General Discussions and Conclusions. -- Overall, the results have shown that the prepared elastomers were rubbery and their mechanical properties can be controlled through manipulations of the chain length of the diol used and the degree of polymer acrylation. As well, the papaverine hydrochloride release rate, from 10% v/v loaded cylinder-shaped matrices, was found to be dependent on the degree of macromer acrylation. Decreasing the degree of acrylation of the prepolymers resulted in reduction in the crosslinking density of the formed elastomeric matrices. Furthermore, it was found that co-formulating papaverine hydrochloride with trehalose increases the release rate without altering the linear nature of the drug release kinetics. The results also have demonstrated that these elastomers, with different degrees of acrylation, showed promising non cytotoxicity results with Mv1Lu cells and received reasonably consistent results between the two common cell viability assays (MTT and ³H-thymidine incorporation). The in vivo implantation results showed that no macroscopic signs of inflammation or adverse tissue reactions were observed at implant retrieval sites. We have shown in vivo justification to confirm the potential of these elastomers as soft-tissue friendly materials as well as the candidacy of these elastomers as biodegradable biomaterials for long term application. We have also demonstrated that elastomer selection can be tailored to achieve the desired implantation period and release rates. Promising results regarding osmotic-driven controlled IL-2 release have been demonstrated via typical zero-order release kinetics. The increase in the device's surface area and the incorporation of trehalose in the loaded lyophilized mixture increased the IL-2 release rate. As well, it was shown that the decrease in the degree of prepolymer acrylation of the prepared devices increased the IL-2 release rate. Cell based bioactivity assay for IL-2 over a release period of 28 days showed that the released IL-2 retained more than 94% of its initial activity.